A variety of processes for converting aromatics in the presence of molecular sieve catalysts are known in the chemical processing industry. Aromatic conversion reactions include alkylation and transalkylation to produce alkylaromatics such as ethylbenzene (EB), ethyltoluene, cumene and higher aromatics. An alkylation reactor which produces a mixture of mono- and poly-alkylaromatic compounds may be linked in some way with a transalkylation reactor to maximize the net production of mono-alkylaromatic compounds. Such alkylation and transalkylation conversion processes can be carried out in the liquid phase, in the vapor phase, or under conditions in which both liquid and vapor phases are present. The preferred catalysts and the byproduct formation differ with the severity of reaction conditions and the phase conditions in which the reaction is carried out.
In efforts to improve commercial alkylation operations, emphasis is placed not only on the conversion efficiency of the catalyst but also on the selectivity of the catalyst, including reduced production of certain byproducts. For example, in the manufacture of ethylbenzene, ethylene and benzene are introduced into an alkylation reactor in the presence of various catalysts. Some of the byproducts include diethylbenzenes, xylenes, propylbenzene, cumene, butylbenzene, and other components referred to collectively as heavies. Production of unwanted byproducts increases feedstock usage as well as the cost of separating such unwanted byproducts. Byproducts which are not removed can materially impact the efficiency of downstream operations, such as the dehydrogenation of EB to form styrene monomer.
It has been shown that zeolites like ZSM-5 show high activity and selectivity for vapor phase alkylation of benzene with ethylene and that catalysts of this type in the acid form remain active for unusually long periods between regenerations. Discussion of acid zeolite ZSM-5 for vapor phase alkylation is provided in U.S. Pat. No. 3,751,506, which is herein fully incorporated by reference and which describes control of the exothermic heat of reaction by conducting the reaction in a series of reactors with intermediate cooling and addition of ethylene between stages.
Another process for vapor phase alkylation is described in U.S. Pat. No. 4,107,224, which is herein fully incorporated by reference. Benzene and dilute ethylene are reacted in vapor phase over a solid porous catalyst selected from ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38, and other similar materials in a series of reaction zones with intermediate injection of cold reactants and diluent to control temperature.
U.S. Pat. No. 6,090,991, which is herein fully incorporated by reference, describes vapor phase ethylbenzene production in which a feedstock containing benzene and ethylene is applied to an alkylation reaction zone having at least one catalyst bed containing a monoclinic silicalite catalyst having a weak acid site concentration of less than 50 micromoles per gram.
U.S. Pat. No. 6,057,485, which is herein fully incorporated by reference, describes vapor phase ethylbenzene production by alkylation over a split load of monoclinic silicalite alkylation catalysts having different silica/alumina ratios. A feedstock containing benzene and ethylene is applied to a multi-stage alkylation reaction zone having a plurality of series-connected catalyst beds. At least one catalyst bed contains a first monoclinic silicalite catalyst having a silica/alumina ratio of at least 275. At least one other catalyst bed contains a second monoclinic silicalite catalyst having a silica/alumina ratio of less than about 275.
U.S. Pat. No. 5,998,687, which is herein fully incorporated by reference, describes ethylbenzene production by alkylation over a stacked reactor loaded with zeolite beta followed by zeolite Y to reduce overall flux oil production.
A disadvantage of vapor phase alkylation reactions is the formation of polyalkylated byproducts. While the art currently provides for various transalkylation processes to handle some of the alkylation byproducts such as diethylbenzene, it would be desirable to reduce the production of byproducts, especially byproducts that are not easily handled in an alkylation/transalkylation process. It would also be desirable to reduce the quantity of reactants consumed in production of byproducts
Recently, catalysts have been developed which allow the alkylation reactions to be carried out in the liquid phase alkylation at relatively mild reaction conditions. The reduced temperature associated with operating in the liquid phase allows for a significant reduction in undesirable by-products.
In existing facilities designed for vapor phase reactions, it can be cost-prohibitive to retrofit for a liquid phase operation unless a substantial increase in production capacity is required. Improved catalysts allowing lower temperature operation in such vapor phase facilities are highly desirable.